Definition:
ISSD is caused when sialic acid, a kind of charged sugar, is unable to be transported out of the lysosomal membrane and instead, sialic acid accumulates in the tissue and free sialic acid is excreted in the urine. ISSD is the most severe form of the sialic acid storage diseases.

ISSD affects 1 in 528,000 live births worldwide.

Symptoms:
Symptoms present by eight months of age and are marked by developmental delay followed by neurological complications such as seizures, involuntary eye movements, and ataxia or involuntary muscle movements. An enlarged liver, spleen, and heart, and coarse facial features are also symptoms.

Inheritance Pattern:
ISSD is an autosomal recessive disorder.

Life Expectancy:
Life expectancy for ISSD is early childhood.

Diagnosis and Testing:
A diagnosis can be made by measuring cultured tissue samples for increased levels of free sialic acid. Prenatal testing is also available for known carriers of this disorder.

Treatment:
There is no treatment for ISSD. Treatment is limited to controlling the symptoms of this disorder such as administering anti-convulsant medication to control seizure episodes.